FIG. 8 is a cross-sectional view schematically showing a pressure-reducing valve 1 according to prior art disclosed in Japanese Laid-Open Patent Application Publication No. 2003-150249. The pressure-reducing valve 1 includes a housing 2, a piston 3, and a spring member 4. In the interior of the housing 2, the piston 3 is axially displaceably mounted. In the interior of the housing 2, the piston 3 is axially elongated and is retained at an axial end portion and at an intermediate axial portion. The spring member 4 applies a spring force to the piston 3 along the axis of the piston. The housing 2 is provided with a primary port 5 and a secondary port 6. A protrusion 7 is formed to surround the outer periphery of the primary port 5. The protrusion 7 and a seat portion 8 of the piston 3 that is opposite to the protrusion 7 forms an orifice 9 for reducing pressure. The orifice 9 separates the interior of the housing 2 into a primary-pressure space 10 connected to the primary port 5 and a secondary-pressure space 11 connected to the secondary port 6. The pressure-reducing valve 1 reduces a primary pressure p1 of fluid supplied to the primary port 5 to a secondary pressure p2 by passing it through the orifice 9, and outputs the resulting fluid from the secondary port 6.
Since the prior art pressure-reducing valve 1 is constructed such that the elongated piston 3 is retained at the first axial end portion and at the intermediate axial portion, the following two problems arise.
First, if retaining portions 12 and 13 for retaining the first axial end portion and the intermediate axial portion of the piston 3 are disposed coaxially with low precision, then a non-uniform contact surface pressure is applied to the piston 3 in the circumferential direction. To be specific, the piston 3 is retained in an uneven contact condition in the interior of the housing 2. Thereby, an undesired friction force is applied to the piston 3, causing hysteresis to be generated in a pressure-reducing characteristic. The hysteresis of the pressure-reducing characteristic increases as the friction force applied to the piston 3 increases, making it difficult to control the pressure-reducing valve 1.
Second, to solve the above stated problem, it is necessary to improve the precision with which the retaining portions 12 and 13 are disposed coaxially. In order to improve the precision with which the retaining portions 12 and 13 are disposed coaxially, it is necessary to improve processing precision of the housing 2, i.e., decrease a coaxial tolerance. This leads to increased cost necessary to manufacture the housing 2. As a result, it becomes difficult to manufacture the pressure-reducing valve 1 in large quantities.